An example dual fuel engine is an engine that includes a first fuel source that is utilized as the sole fuel source during certain operating conditions and a second fuel source that is integrated with the first fuel source at other operating conditions. In certain applications, the first fuel source is a diesel fuel and the second fuel source is natural gas. The diesel fuel provides, in some cases, the initial, low load levels of operation and is used for ignition for the natural gas at higher load operations. The substitution of natural gas for diesel fuel reduces the cost of operating the engine, particularly when the engine is employed at a location where natural gas is abundant or available at low cost.
When the engine is operating in dual fuel mode, natural gas fuel is introduced into the intake system. The air and natural gas mixture from the intake is drawn into the cylinder, just as it would be in a spark-ignited engine, but the air-to-fuel ratio of the charge mixture can be much leaner than a typical spark-ignited engine. Diesel fuel is injected near the end of the compression stroke, just as it would be in a traditional compression-ignition engine. The diesel fuel is ignited by energy compression heating of the charge and the energy released from combustion of the diesel fuel causes the natural gas to burn. A dual fuel engine can operate either entirely on diesel fuel or on the substitution mixture of diesel and natural gas, but generally cannot operate on natural gas alone except where an auxiliary ignition source is provided to the cylinder.
Dual fuel engines encounter difficulties during operation due to different compositions and variations in the natural gas that may be used for engine operation and in site conditions where the engine may be operated. In addition, existing control schemes have difficulty in achieving high substitution rates of natural gas for diesel fuel over a wide range of operating conditions and natural gas quality. While some control strategies compensate for natural gas quality variations by controlling gas substitution to obtain a desired air-to-fuel ratio, such approaches are limited by the ability to accurately measure and/or predict air flow rates, fuel properties, and sometimes gaseous fuel flow rates.
Other combustion strategies, such as pilot ignited gaseous fuel engines, utilize the diesel fuel only for pilot ignition of the gaseous fuel and air mixture. With this approach, the mixture of air and gaseous fuel must be carefully controlled to sustain combustion throughout the cylinder and away from the small diesel pilot ignition source. Extremely high substitution rates can be achieved, but these systems require intake air throttling and control in order to actively maintain required air-to-fuel ratios, and are limited to lower boost pressures, lower compression ratios, and lower power density than what can be achieved with unthrottled dual fuel compression ignition engine operation. In addition, the intake air throttling required for pilot-ignited gaseous fuel engines reduces the overall efficiency of the engine as compared to an unthrottled engine. Dual fuel engines are able to run with extremely lean charge air and gaseous fuel mixtures because the diffusion combustion of the diesel fuel creates turbulent regions of extremely high temperature within the cylinders that can drive oxidation reactions of the gaseous fuel even when the mixture is sol lean that it could not sustain the chain reactions or propagate a flame front from the ignition source.